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e) The main gas valve is then energized to allow
gas to mix with the combustion air.
f) If the main flame is not detected in 5 seconds,
the gas valve closes and the combustion fan goes
into post-purge. Otherwise, the combustion fan
stays at the ignition speed for the length of the
flame stabilization period. (10 to 25 seconds).
g) After the flame stabilization period is complete,
the combustion air fan ramps down to its low fire
setting. Then the boiler will begin to modulate
depending on the supply temperature to
maintain the set point temperature.
h) As the supply temperature approaches the set
point, the combustion control will reduce the
input rate. The supply temperature will continue
to climb to about 7°F above the boiler set point
before it cycles off.
i) The differential is applied from this point. For
example if the boiler set point is at 180°F with a
10°F differential, the supply temperature will
reach 187°F before shutting down. It will not
re-start until the supply temperature drops below
177°F.
Figure 6.3: Boiler Control Board Wiring Schematic